Exercise in the heat: Individual impacts of heat acclimation and exercise training on cardiovascular performance

1. The individual contributions of heat acclimation and exercise training to improving cardiovascular reserves during exercise under different environmental conditions [normothermia: 24°C, 40% relative humidity (RH); hot/wet: 35°C, 70% RH; hot/dry: 40°C, 20% RH; and work loads mild: 15 m min^-1; moderate; 25 m min^-1) were studied in chronically cannulated conscious rats. 2. Heat acclimation elicited a decrease in blood pressure before and during the exercise (20 min). Both heat acclimation and training induced bradycardia, although the training effect was more pronounced. During the exercise acclimated rats showed a delayed increase in heart rate compared with that of non-acclimated rats. 3. The results suggest that both heat acclimation and exercise training increase cardiac efficiency, as demonstrated by the double product index. This was achieved, however, in different ways by the trained and the acclimated rats.